1. Field of the Invention
The present invention relates to an electron beam exposure technique in a pattern exposure process in the manufacture of semiconductor elements and, more particularly, to an electron beam projection transfer exposure apparatus which assures high use efficiency of electron beams and has high productivity.
2. Description of the Related Art
In the manufacturing process of semiconductor memory elements, light exposure that can assure high productivity is used. However, in the manufacture of semiconductor memory elements such as 1G and 4G DRAMs, which require processing an exposure pattern having a minimum line width of 0.2 .mu.m or less, electron beam exposure with higher resolution is beginning to gain attention as an alternative exposure technique to light exposure.
In recent years, a transfer electron beam exposure apparatus which aims at improving productivity as compared to a conventional Gaussian or variable shaped electron exposure apparatus, i.e., an exposure apparatus which irradiates an electron beam onto a mask surface formed with a desired pattern and transfers the electron beam transmitted through the mask in a reduced scale to project the pattern on the mask onto a wafer, has been developed.
FIG. 7 shows the arrangement of a conventional electron beam projection exposure apparatus. In the conventional electron beam projection exposure apparatus shown in FIG. 7, an electron beam emitted by an electron gun 101 is condensed by a condenser lens 102 formed with a rectangular aperture 121, and uniformly illuminates the surface of a mask 104 via a field lens 103. As the mask 104, either a scattering type mask prepared by forming a scattering pattern for scattering the electron beam on a membrane that transmits the electron beam, or a stencil type mask prepared by forming an absorbing pattern for absorbing the electron beam on a membrane may be used. The electron beam which is neither scattered nor absorbed by the pattern on the mask 104 is imaged and irradiated on a wafer 107 via reduction projection lenses 105 and 106, thus forming an image of the pattern on the mask 104 onto the wafer 107 by exposure. Since the electron beam is transmitted through the rectangular aperture 121, an electron beam irradiation region 110 on the wafer 107 also has a rectangular shape, as shown in FIG. 8. The mask 104 is mounted on an X-Y stage which can be scanned in the X- and Y-directions, and the wafer 107 is mounted on a wafer stage. By scanning the X-Y stage in synchronism with the wafer stage, the entire surface of the wafer 107 is exposed.
One of the major factors that determine the productivity of the above-mentioned electron beam projection exposure apparatus is the exposure area (irradiation area) of an electron beam on the wafer surface. Especially, the exposure width (stripe width) W (see FIG. 8) of an electron beam in a direction perpendicular to the scanning direction of the wafer stage is an important factor. The exposure area is determined by off-axis aberrations of a projection lens of an electron optical system. In the manufacture of 1G or 4G DRAMs, the allowable value of the blur amount of an electron beam on the exposure surface is 20 to 30 nm. Hence, the allowable value of the blur amount due to aberrations of the electron optical system must be set to be equal to or smaller than this value, and a status quo numerical value example of the exposure area is around 0.25 mm.times.0.25 mm. The throughput in this case is five 8" wafers/hour, and sufficient performance is not obtained in terms of productivity (J. Alexander Liddle et al., Jpn. J. Appl. Phys. Vol. 34 (1995) pp. 6663-6671).
As for the intensity of the electron beam, the electron gun requires an emittance of around 10.sup.-5 to 10.sup.-4 (cm.cndot.rad) if off-axis aberrations can be reduced and the exposure area is several ten mm square. This value is one to two orders of magnitudes larger than that obtained by a LaB6 emitter electron gun used in the electron beam exposure apparatus. Therefore, when the exposure region is broadened while prescribed current density conditions of the electron gun determined by the resist sensitivity and stage scanning speed are satisfied, the intensity of the electron beam is limited by the emittance of the electron gun. Hence, a sufficiently high electron beam intensity cannot be obtained if the exposure region is broadened.
As described above, in the conventional electron beam exposure apparatus, it is hard to broaden the exposure region while maintaining the electron beam intensity required for obtaining a throughput with high productivity.